Quantifying weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl: a combined computational and experimental investigation of NMR chemical shifts in the solid state.
Quantifying weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbiphenyl: a combined computational and experimental investigation of NMR chemical shifts in the solid state.
复制标题
量化尿嘧啶和 4-氰基-4-乙炔基联苯中的弱氢键：固态 NMR 化学位移的计算和实验相结合的研究。
DOI：
10.1021/ja075892i
复制
发表时间：
2008
影响因子：
15
通讯作者：
Uldry AC
中科院分区：
文献类型：
--
作者：
Uldry AC
利用魔角自旋固体核磁共振（MAS）测定1H、13 C和15 N的化学位移，结合平面波基组第一性原理计算，研究了尿嘧啶和4-氰基-4 '-乙炔基联苯中的弱氢键，其单晶衍射结构显示CH···OC和C <$CH···N <$C距离很近.本文介绍了一条合成天然丰度的4-氰基-4 '-乙炔基联苯的优化路线，包括中间体的分离和表征。化学位移计算的差异，一方面，为完整的晶体结构，另一方面，为一个孤立的分子取决于两个分子间的氢键相互作用和芳环电流效应。在这项研究中，这两种效应是分开计算，首先，确定化学位移之间的差异计算的平面（尿嘧啶）或一个孤立的链（4-氰基-4 '-乙炔基联苯）和计算的一个孤立的分子，第二，计算面内或链内的核独立的化学位移，量化的环电流效应所造成的相邻分子。对于尿嘧啶，在CH···O弱氢键中涉及的CH质子，1H化学位移的2.0和2.2 ppm的分离分子到平面的变化被确定;相比之下，在常规NH···O氢键中涉及的NH质子的变化为5.1和5.4 ppm。比较CH键长的几何松弛的尿嘧啶分子的晶体结构和几何松弛的孤立的分子显示不超过0.002毫米，这对应于在计算的1H化学位移的变化，最多0.1 ppm的差异。对于4-氰基-4 ′-乙炔基联苯中的C → CH···N → C弱氢键，计算的分子链变化量与供体13 C和受体15 N核的分子链变化量大小相似，但符号相反。在尿嘧啶和4-氰基-4 ′-乙炔基联苯中，CH氢键供体分别为sp2和sp3杂化;计算的1H化学位移变化与麦芽糖中sp3杂化CH供体的变化的比较（Yates等人，J.Am.Chem.2004，1999）。Soc.2005，127，10216）揭示了对于弱氢键强度，杂交没有显著的依赖性。
Weak hydrogen bonding in uracil and 4-cyano-4‘-ethynylbiphenyl, for which single-crystal diffraction structures reveal close CH···OC and C⋮CH···N⋮C distances, is investigated in a study that combines the experimental determination of1H,13C, and15N chemical shifts by magic-angle spinning (MAS) solid-state NMR with first-principles calculations using plane-wave basis sets. An optimized synthetic route, including the isolation and characterization of intermediates, to 4-cyano-4‘-ethynylbiphenyl at natural abundance and with13C⋮13CH and15N⋮C labeling is described. The difference in chemical shifts calculated, on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule depends on both intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the two effects are separated computationally by, first, determining the difference in chemical shift between that calculated for a plane (uracil) or an isolated chain (4-cyano-4‘-ethynylbiphenyl) and that calculated for an isolated molecule and by, second, calculating intraplane or intrachain nucleus-independent chemical shifts that quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to plane changes in the1H chemical shift of 2.0 and 2.2 ppm are determined for the CH protons involved in CH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons involved in conventional NH···O hydrogen bonding. A comparison of CH bond lengths for geometrically relaxed uracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differences of no more than 0.002 Å, which corresponds to changes in the calculated1H chemical shifts of at most 0.1 ppm. For the C⋮CH···N⋮C weak hydrogen bonds in 4-cyano-4‘-ethynylbiphenyl, the calculated molecule to chain changes are of similar magnitude but opposite sign for the donor13C and acceptor15N nuclei. In uracil and 4-cyano-4‘-ethynylbiphenyl, the CH hydrogen-bonding donors aresp2andsphybridized, respectively; a comparison of the calculated changes in1H chemical shift with those for thesp3hybridized CH donors in maltose (Yates et al.J. Am. Chem. Soc.2005,127, 10216) reveals no marked dependence on hybridization for weak hydrogen-bonding strength.